Experimental Studies of the Interaction of Atmospheric Aerosol Particles with Clouds and Fogs

In this work, cloud and fog droplet formation was studied in three joint field experiments. The instrument, the droplet aerosol analyser (DAA), has also been further developed and verified as part of this work. The DAA is an instrument especially developed for studies of cloud and fog droplet formation and growth. It measures the ambient size of individual droplets and interstitial particles in a fog or a cloud. It then measures the size of the individual dry residual particles after evaporation of the water. It also counts the number of dry particles of each size, which gives a unique three-parameter data set that connects ambient size to dry size and to the number of particles. Having access to these parameters, a number of related aerosol/cloud parameters can be determined, whereof the microstructure, i.e. the size and number of droplets and their size distribution, characterisation of the droplet activation, as defined by the Köhler equation, and the size-dependent cloud droplet nucleation scavenging of particles due to activation, are the most important.

The results from one fog experiment showed that, most of the time fog consisted of unactivated droplets with a continuous size distribution in the size region 1-47 µm in diameter. There were no gaps generated by the droplet activation process, as has been observed in clouds. In the two ground-based cloud experiments, cloud droplet number concentrations increased almost linearly with particle number concentration. Droplet concentrations of up to 2000 per cubic centimetre in an experiment in northern England and up to almost 3000 per cubic centimetre in an experiment on Tenerife, Spain, were observed, thus demonstrating the influence of air pollution on cloud microstructure.